1. Technical Field
This invention relates to high-pressure discharge lamps having a discharge vessel enclosed by an outer bulb provided with a lamp cap, and more particularly to a lamp having a starting aid arranged in the intervening space between the outer bulb and the discharge vessel.
2. Description of the Related Art
High pressure discharge lamps, or more particularly metal halide lamps, having starting aids are known in the art. Such lamps are suitable for various applications such as general interior lighting, general exterior lighting, video illumination, etc. The discharge vessel of the known lamp is typically made of quartz glass. Alternatively, this vessel may be made of a ceramic material. Ceramic material in the present description and claims is understood to be a densely sintered polycrystalline metal oxide such as, for example, Al.sub.2 O.sub.3 or YAG and densely sintered polycrystalline metal nitride such as, for example, AlN.
A known problem of metal halide lamps is the comparatively wide spread in ignition time. This problem arises from a shortage of free electrons due to the presence of electronegative iodine in the lamp filling. Several methods are known in the art to counteract this problem. For example, the addition of a small quantity of .sup.85 Kr in the discharge vessel can supplement such a shortage. A disadvantage of .sup.85 Kr as a filling material is its radioactive characteristics.
Alternatively, ignition aids, such as a UV-enhancer, are used in metal halide lamps to promote ignition. A UV-enhancer is typically a small discharge tube positioned adjacent the discharge vessel that acts as an ultraviolet radiation source. Such a UV-enhancer has been disclosed in U.S. Pat. No. 4,818,915 to Zaslavsky et al. This UV-enhancer has an envelope of UV-transmitting quartz material. Upon breakdown, the UV-enhancer will generate UV-radiation at about 253.7 nm or less. The influence of this UV-radiation leads to the production of free electrons in the discharge vessel, which in their turn strongly promote lamp ignition.
The use of the quartz UV-enhancer in the known lamp leads to an improvement in situations where ignition voltage pulses of the order of 5 kV are useful and admissible. Under many circumstances occurring in practice, however, it is desirable or even required that the ignition voltage pulses should not substantially exceed a level of 3 kV. In addition, the manufacture and dosing of such UV-enhancers is complex and expensive.
Another starting aid is known from commonly-assigned U.S. Pat. No. 5,811,933 to van de Niewenhuizen et al. The lamp disclosed therein is characterized in that the wall of the UV-enhancer is made from ceramic material. The probability of breakdown upon the application of an ignition pulse rises strongly both in the UV-enhancer and in the discharge vessel owing to the presence of the ceramic material in the wall of the enhancer. The increased breakdown probability manifests itself in a drop in the minimum ignition pulse value required for a reliable lamp ignition.
The use of the UV-enhancer as disclosed in the above lamp leads to an improvement in ignition characteristics. However, the manufacture of the completed ceramic enhancer itself may require the use of costly parts and materials as well as additional manufacturing steps. In the lamp disclosed above, the UV-enhancer is constructed from ceramic material which has been extruded into hollow cylinders and sintered to achieve the necessary translucency and gas-tight characteristics. As compared to quartz, the ceramic material cannot be softened and reworked after sintering. The resulting open end portions of the cylindrical sections must be sealed to contain the filling. Consequently, an additional part, i.e. an end plug would be needed. An additional manufacturing (or process) step is required to seal the end of the enhancer tube. It is therefore desirable to improve the operation and manufacture of the lamp by reducing the material and manufacturing steps necessary.
It desirable to use the UV-enhancer in smaller lamp applications. However, a construction that requires smaller multiple individual pieces makes such miniaturization difficult. It is therefore desirable from this point of view to provide a starting aid having simplified construction.
The integrity of the seals is critical to the efficacy and lifetime characteristics of the UV-enhancer and the lamp in general. The use of different materials having different thermal expansion and durability characteristics may detract from the operation of the lamp. It is therefore desirable to improve the integrity of the seals by simplified construction.